226 TROPIC MOVEMENTS 



the cells of a displaced root the protoplasm accumulated at the points from 

 which the starch-grains had moved, but this was probably the direct result 

 of the displacement of the starch. 



Jensen 1 supposes that, in 'the case of freely motile organisms, their 

 geotactic irritability is the result of their response to the differences of 

 pressure at varying depths, which enable them to orient themselves in 

 regard to the perpendicular. Jensen forgets, however, that the maximal 

 differences of pressure capable of affecting the organism at a given time are 

 exceedingly small. On the other hand, the gravitational acceleration is 

 only constant so long as the organism is moving with uniform velocity 

 along a straight path, which is never the case. Every time the velocity 

 changes, or the direction of motion alters, the organism experiences an 

 increase or decrease of the geotropic stimulus. These changes, though 

 relatively feeble, might well act as directive stimuli. 



Owing to the subordination of the individual cells in each tissue or 

 organ their potential powers of sensation and response are not always fully 

 represented in every response. Hence when the growth in length of a 

 curving radial organ is accelerated on the convex side, retarded on the 

 concave one, and unaffected in the middle lamella, this does not justify 

 Noll's conclusion that a corresponding distribution of sensibility is involved 

 in the responsive cells ' 2 . A precisely similar distribution of the growth- 

 activity is shown in curving unicellular organs, and a tissue composed of 

 such cellular organs would undoubtedly show the same differences of 

 growth, for a tendency to curvature on the part of the individual cells can 

 only find external expression when the rate of growth of convex and 

 concave sides undergoes appropriate alteration. 



It is therefore impossible to follow Noll, or even Nemec and Haber- 

 landt, in ascribing the realized reaction to the unequal distribution of 

 irritability in the individual cells, or in their radial and longitudinal walls. 

 Nor does it follow that the different cells of a Pandorina possess dissimilar 

 irritabilities because they are at varying angles with the incident rays when 

 the colony is phototactically oriented. In plagiotropic positions the starch- 

 grains collect at the lower corners of the cells, but this does not afford any 

 explanation of the plagiotropic irritability, as Nemec supposes 3 . There 

 can be no doubt that, as in the case of tendrils, each organ responds as 

 a whole to geotropic excitation, but the regulation of the individual cells 

 is probably an extremely complex phenomenon. Czapek 4 has attempted 

 to explain this regulation as being due to the pressures and stresses which 



1 Jensen, Bot. Centralbl., 1893, Bd. LVI, p. 21. 



2 Noll, Heterogene Induction, 1892, p. 31. Cf. also Jost, Biol. Centralbl., 1902, Bd. xxn, 

 p. 169 ; Haberlandt, Ber. d. hot. Ges., 1903, p. 470 ; Nemec, Ber. d. bot. Ges., 1902, p. 359. 



3 Nemec, 1. c., 1901, p. 310. 



* Czapek, Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 236; Ber. d. bot. Ges., 1901, p. 123. 



